The present invention relates to a process for the preparation of phosphonomethylglycine from N-phosphonomethyliminodiacetic acid N-oxide.
Phosphonomethylglycine, which is known under the common name glyphosate, is a potent phytotoxic compound which is employed as herbicide.
EP-A-439445 describes the preparation of phosphonomethylglycine starting from N-phosphonomethyliminodiacetic acid. The latter is oxidized with a peroxide in aqueous solution, if appropriate in the presence of a catalytically effective amount of a water-soluble molybdenum compound, to give the intermediate N-phosphonomethyliminodiacetic acid N-oxide. The N-oxide is subsequently converted into phosphonomethylglycine in the presence of a catalytic amount of a metabisulfite compound and a water-soluble molybdenum compound.
EP-A-464017 also describes a process for the preparation of phosphonomethylglycine starting from phosphonomethyliminodiacetic acid using the same process steps. The oxidation to give the abovementioned N-oxide is carried out with a peroxide in the presence of a water-soluble molybdenum or tungsten compound. The conversion into phosphonomethylglycine is then carried out using iron, zinc, aluminum, vanadium or copper in the form of the metal or using a vanadium salt, iron(II) salt or copper(I) salt as catalyst.
Finally, EP-A-464018 also describes a process for the preparation of phosphonomethylglycine, the oxidation of the phosphonomethyliminodiacetic acid being carried out with a peroxide in the presence of a water-soluble tungsten compound or of a mixture of a water-soluble tungsten and molybdenum compound as catalyst. The N-oxide is then brought into contact with metallic iron, a water-soluble vanadium compound, an iron(II) salt or a mixture of a water-soluble sulfide, sulfite or bisulfite compound and a water-soluble molybdate compound and is converted into phosphonomethylglycine.
Thus, readily reducible catalysts are employed when the N-oxide is converted into phosphonomethylglycine. Since peroxide is generally employed in an excess for oxidizing the N-phosphonomethylimino-diacetic acid, the result is a reaction between the excess peroxide and the readily oxidizable catalyst, which is thus inactivated, at least to some extent.
It is an object of the present invention to provide a process for the preparation of phosphonomethylglycine which rapidly yields phosphonomethylglycine in high yield and high selectivity, even in the presence of residual oxidant.
We have surprisingly found that this object is achieved when the oxidation of N-phosphonomethyliminodiacetic acid N-oxide is carried out with thiocyanic acid or a salt thereof as catalyst.
The present invention therefore relates to a process for the preparation of phosphonomethylglycine wherein N-phosphonomethyliminodiacetic acid N-oxide is brought into contact with a catalytically effective amount of thiocyanic acid or a salt thereof, if appropriate in the presence of a cocatalyst.